A Siege of Toulouse occurred from October 1217 to June 1218 during Albigensian Crusade. It was third of a series of sieges of the city during the height of Crusader efforts to put down Catharism (and the local Languedocian nobility). It ended in the repulsion of the Crusaders and the death of their leader, Simon IV de Montfort.

Simon first besieged Toulouse in 1211, but failed to take it. In 1216 he captured the city and proclaimed himself Count of Toulouse, but while he was elsewhere, on 12 September 1217, the legitimate claimaint, Raymond VI, retook the city without a fight. Simon returned in haste to besiege the city once more. Despite that, he had ordered the city's defences dismantled while he was in control, he found its defences intact and its walls well-manned. His forces were too small to surround it and the siege dragged on through the winter months with little activity.

Plaque commemorating the death of Simon de Montfort

In the spring of 1218 a certain maestre (master) Bertran of Toulouse suggested to the people that they construct a trebuchet. The city's carpenters immediately took up the task. Meanwhile, on 3 June, the Crusaders constructed (or had brought in) a "cat" (a leather-covered, steeply-gabled mobile shelter) in order to approach the walls. The defenders' trebuchet swiftly dispatched it and, on 25 June, they sortied to burn the cat, which they did. During the Crusaders' counterassault, Simon stopped to aid his brother Guy, who had been wounded by a crossbow, and was hit on the head by a stone from one of the defenders' siege engines (either the trebuchet or a mangonel), apparently operated by donas e tozas e mulhers (ladies, girls, and women). It killed him. The leadership of the Crusade fell to his son Amaury but the siege was soon lifted.

The events of the siege prompted the resident troubadour (and possibly priest) Raimon Escrivan to compose a song, Senhors, l'autrier vi ses falhida, on it. The song, a tenso, presents a mock debate between two siege machines (the trebuchet and the cat) in which the trebuchet wins.